Improve exceptions messages in code creating stages

app/services/ci/ensure_stage_service.rb

@@ -28,9 +28,11 @@ module Ci
       find_stage || create_stage
     rescue ActiveRecord::RecordNotUnique
       retry if (attempts -= 1) > 0
+
       raise EnsureStageError, <<~EOS
-        Possible bug in the database load balancing detected!
-        Please fix me!
+        We failed to find or create a unique pipeline stage after 2 retries.
+        This should never happen and is most likely the result of a bug in
+        the database load balancing code.
       EOS
     end
 

db/post_migrate/20180119121225_remove_redundant_pipeline_stages.rb

@@ -11,9 +11,12 @@ class RemoveRedundantPipelineStages < ActiveRecord::Migration
     add_unique_index!
   rescue ActiveRecord::RecordNotUnique
     retry if (attempts -= 1) > 0
+
     raise StandardError, <<~EOS
       Failed to add an unique index to ci_stages, despite retrying the
-      migration 100 times. See gitlab-org/gitlab-ce!16580.
+      migration 100 times.
+
+      See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16580.
     EOS
   end